1. Technical Field
Systems and methods consistent with the present invention relate to time-slotted optical burst switching for smoothly supporting a data transmission service for a constant bit rate and a variable bit rate in an optical burst switching network.
2. Related Art
Optical communication is superior to other communication methods due to its availability in a broad frequency region. Assuming that loss per unit distance is 2 dB/km, the bandwidth is about 130 THz (100 nm) for optical communication. Among such a broad transmissible region, a current optical communication technology has used only a single channel having several hundreds MHz to several GHz bands near 1.3 μm wavelength.
A wavelength division multiplexing transmission scheme is based on a concept of utilizing such a broad band to the maximum, and uses 1310 nm and 1550 nm regions. In the related art, channels are arranged by a predetermined wavelength interval near the wavelength of the 1550 nm region, and signals are loaded on each channel. Also, a number of channels are optically multiplexed, and the signals are transmitted through an optical fiber. A receiver optically demultiplexes the channels according to their wavelengths and utilizes each channel separately.
A related art router connects one network component to another network component by transferring a packet between an input link and an output link. Such a connection is accomplished by transferring the packet to a next node on an optimized path which is extended to a destination according to an address included in the inputted packet.
A related art optical burst switching (OBS) scheme and a related art optical packet switch (OPS) is provided in optical packet routing schemes. In the OBS, a length of the data packet can be variable and a packet routing can be performed without an optical buffer by setting a path in advance using a control packet. In OPS, routing is performed in the optical packet unit having a fixed length with header information.
Generally, the optical fiber additionally requires a photoelectric converter for converting an optical signal into an electrical signal and an electrooptic converter for converting an electrical signal into an optical signal, which results in an increased cost. In order to solve such a problem, there was proposed in the related art an optical burst switch, which does not convert the transferred optical signal into the electrical signal but processes the optical signal directly.
In the related art optical burst switching network, generally, asynchronous transfer mode (ATM) or Internet protocol (IP) packets inputted in an optical domain are gathered as a data burst in an edge node, and such data bursts are routed by way of a core node depending on their destinations or Quality of Services (QoS) and then sent to the destination nodes. Further, a burst control packet (BCP) and the data burst are respectively transmitted on different channels and at an offset time. That is, the burst control packet is transmitted earlier than the data burst by the offset time and it previously reserves a path through which the data burst is transferred, so that the data burst can be transmitted through the optical network at a high speed without being buffered.
However, since the packet size is variable in the related art optical burst switching scheme, data burst loss frequently occurs due to a contention in the optical switch.
The related art optical burst switching scheme uses two kinds of mechanisms to reduce the contention on output channels, that is, wavelength conversion and an optical buffer.
A time-slotted OBS is a scheme capable of reducing burst loss using a fiber delay line (FDL) buffer only. This related art scheme reduces the possibility of burst loss, regardless of the number of channels connected to one link.
The time-slotted OBS reduces the data burst loss in the core node by setting the data bursts receiving in the core node to a predetermined size and synchronizing them, so that it enhances optical switching efficiency.
Hereinafter, a related art procedure of transmitting optical data will be described with reference to FIG. 1. FIG. 1 is a block diagram showing a time-slotted optical burst switching network in the related art. Hereinafter, a procedure of transmitting a data burst in a time-slotted OBS network 100 will be described.
Node A 101 and node B 103 are both edge nodes gather packets, and make data bursts when receiving ATM or IP packets as inputs, the data bursts having a predetermined length, and transmit the data bursts using the same time slot. The edge nodes 101, 103, 109 and 111 perform a function of making an optical data burst packet by gathering packets and transmitting the optical data burst packet. Further, the edge nodes 101, 103, 109, 111 perform a function of receiving the optical data burst packet or dividing it into individual packets.
Node C 105 and node D 107 are core nodes that perform a function of optically switching the optical data burst. The node A 101 or the node B 103 generates burst control packet BCP and transmits it to the core node, i.e., the node C 105 when data bursts of one or more slots are generated. Further, node A 101 or node B 103 transmits the data bursts to node C 05 after an offset time. The burst control packet includes information on a destination address of the data burst, a source address of the data burst, a size of the data burst, QoS of the data burst, and an offset time of the data burst.
Node C 105 identifies a destination address of the data to be received hereafter, determines an optical path and schedules time for the optical switching using the transferred burst control packet. In node C 105, while the burst control packet is converted from the optical signal to the electrical signal or vice versa, the data burst goes through the optical path by performing an optical switching without the optoelectric conversion. Node C 105 can switch the data burst to node E 109 or node D 107 depending on whether the destination of the burst transmitted from node A 101 is node E 109 or node F 111.
The procedure where node C 105 transfers the data burst transmitted from node A 101 to node E 109 or node F 111 was explained above. However, node C 105 may be a destination of the data bursts generated from node A 101, or directly generate a data burst to be transmitted to node E 109 or node F 111. That is, node C 105 is a core node that may also have a function of an edge node.
Such a related art time-slotted OBS network 100 can be connected to other optical or electrical networks (not shown). However, there are difficulties in that the time-slotted OBS network 100 deals with services of a constant bit rate (CBR) and a variable bit rate (VBR).
The constant bit rate is used in a connection apparatus that depends on a precise clock processing scheme to guarantee undistorted data transmission with a fixed transmission rate. Properties such as a service band and required delay should be maintained in a predetermined range among the service performance period. Accordingly, a voice signal of a telephone network and the like are typically serviced at a constant bit rate.
The variable bit rate is divided into a real time (RT) class and a non-real time (NRT) class. A VBR RT is used in a connection having a fixed timing relation between samples. Although the VBR NRT has no fixed timing relation between samples, a guaranteed QoS is still used in a required connection. Generally, TV broadcast, IP and file transmission are serviced at a variable bit rate.
As for services of the constant bit rate, the related art time-slotted OBS network has a difficulty since the data burst is generated and the burst control packet is transmitted. That is, since even the burst packet that is arrived at node A 101 and formed in the constant bit rate can be transmitted after the burst control packet is transmitted and then a predetermined time is lapsed, a delay inevitably occurs.